A MOS image pick-up device denotes an image sensor that amplifies and reads signals of respective pixels by using amplifying circuits including MOS transistors formed on the respective pixels. Recently, such MOS image pick-up devices, particularly, so-called CMOS (complementary MOS) image sensors manufactured in a CMOS process, have been marked as image-inputting devices in portable apparatuses such as miniature cameras for PCs, due to the merits, e.g., they require low voltages and consume less power, and the sensors can be integrated with peripheral circuits so as to form a one-chip device.
In a conventional MOS image pick-up device, every circuit in the peripheral circuit region is designed using a CMOS technique to apply both n-channel MOS transistors and p-channel MOS transistors. In the imaging region, all of the MOS transistors composing each pixel are n-channel MOS transistors. In general, the n-channel MOS transistors composing the pixel are identified with n-channel MOS transistors used in the peripheral circuit region.
FIG. 4 is a cross-sectional view showing a structure of a CMOS transistor used for a peripheral circuit region of a conventional MOS image pick-up device. In a semiconductor substrate 21, an n-type well 26 and a p-type well 25 are formed. A p-channel MOS transistor 22 is formed in the n-type well 26, and an n-channel MOS transistor 23 is formed in the p-type well 25. These transistors are isolated electrically from each other by device-isolation portions 24. The device-isolation portions 24 are oxide films 27 formed by LOCOS (local oxidation of silicon). For further miniaturization, oxide films 28 formed by STI (shallow trench isolation) are used for the electron-isolating portions as shown in FIG. 5.
Since the MOS image pick-up device includes an amplifying circuit in each pixel as described above, it can amplify weak signals so as to realize high sensitivity. On the other hand, when a large amount of current leaks into the photodiode, the leakage current may be amplified and cause a considerable noise.
In the above-described conventional MOS image pick-up device, n-channel MOS transistors composing the pixel are regarded as identical in the structure to the n-channel MOS transistors used in the peripheral circuit region, i.e., n-channel MOS transistors of a CMOS transistor. Moreover, the device-isolation portions among the transistors are considered to have a common structure in the imaging region and the peripheral region.
However, the CMOS transistors used for the peripheral circuit region are developed in a trend for miniaturizing semiconductor LSI. Therefore, the main object in development of the CMOS transistors is a high-speed process, while substantially no attention is paid for leakage current. For example, in a CMOS transistor shown in FIG. 4, the oxide films 27 used for the device-isolation portions are formed by thermally oxidizing the substrate 21, and thus they erode the substrate 21 by approximately half of the film thickness. In the CMOS transistor shown in FIG. 5, the oxide films 28 used for the device-isolation portions fill trenches formed on the substrate 21, so that the films erode the substrate 21 by their entire thickness. Since the substrate will be subjected to a great stress at parts eroded by the oxide films in the device-isolation portions, a large leakage current will occur. When the device-isolating structure of the CMOS transistor is used for an imaging region, noise due to the leakage current will be increased considerably.